Mark Gill, director of vessel traffic services for the Coast Guard, tells Minnesota Public Radio News (http://bit.ly/1mK972v) about 60 ships are waiting to enter Lake Superior, which is still 60 percent ice covered.

Three heavy ice breakers are escorting convoys of five ships across the lake, where stacks of wind-blown ice are still 8 feet thick in places.

The season's first trip from Duluth to lower Lake Michigan took two weeks. It normally takes less than three days. Some steel mills and power plants around the Great Lakes have run low on supplies of iron ore and coal.

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